Method for Controlling the Quality of a Gas Produced from Multiple Wells Collected by a Gathering System

ABSTRACT

Using constant pressure to maintain the quality of gas produced from a gathering system. In a gas gathering system where multiple producing wells are feeding a closed system, a change in flow rate/producing pressure at any single well location will have an effect on the producing quality of the other wells on the closed system if the blower/compressor is not adjusted accordingly to compensate for the change. This method automatically adjusts a control unit so that constant pressure is maintained to maintain the quality of the gas.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to controlling and maintaining a desiredquality of a gas by controlling the producing pressure of a group ofwells in a gathering system.

2. Description of Related Art

Currently, a well tender monitors the individual quality of a gas,adjusts the amount of volume of gas from the individual well, and thenhas to adjust the pressure. This practice is time consuming and cancause the well tender to have to constantly move back and forth betweenthe two areas or locations.

There are compressors which are capable of self-regulating the amount ofpressure being produced, however, these compressors are not beingutilized to maintain the quality of gas produced from a group of gases.

BRIEF SUMMARY OF THE INVENTION

Subterraneous formations often contain desirable materials which can beused for many applications. Therefore, there is a need to remove thedesirable materials from the subterranean formation. The subterraneanformations often extend horizontally over many thousands of feet, andare often very shallow in depth.

Coal is a desirable material contained in subterranean formation.Methane is also contained in the subterranean formation. When the coalits mined methane gas is released. When the coal is mined, air isintroduced into the methane gas (or into the mine), to provide adequateventilation/atmosphere for the coal miners.

Wells are drilled into the subterranean formation in order to obtain themethane gas for commercial production. There can be hundreds of wellsspread throughout a mining formation.

The methane gas produced by the wells is used for various endeavors,including sales to commercial energy suppliers. Wells of this nature(mining related in both active and sealed mines), typically have verylow producing pressures as the main seam of coal has been removed, thusleaving a void. This void causes a strain on the upper strata whichleads to breakage and fracturing from the void extending, in someinstances, all the way to the surface of the ground above. Thisfracturing of the upper strata creates pathways for gases in the uppercoal seams (seams above mined seams) to be released. Since the reservoirpressures of the fractured coal seams can reach well into the hundredsof pounds, it is common practice to provide an alternate exit for thegas other than down into the mine. This is done for safety reasons, asMethane gas has an explosive range of 5% to 15% by volume meothane toair concentration. A well is drilled through all the seamus of coal sothat the gas can be produced on the surface for sale, as well as toprevent the gas from escaping into the mine via the fractures.Especially in active mining areas, the pressures on both the surface andinside the mine are very close to the same.

Each of these wells is connected to a central pipeline. As stated, theproducing pressures of the well are typically low, so measures must betaken to compress the gas to higher pressures for sale. The more a gasis compressed, the less space it takes up and the smaller the pipelinecan be used to transport the product. Since often times it is noteconomical to compress the gas produced from each well at eachindividual well location, a network or a grouping of several wells maybe gathered into one central location and compressed.

After the well is compressed is often directed to a transmission linewhere it sold to commercial energy suppliers. The energy suppliers oftenrequire the gas to have a minimum BTU level.

Very slight changes in a producing pressure of a well that produces gasfrom both activate and sealed gob area of a coal mine can have a verylarge effect on the both the volume and the quality of the gas produced.Sealed gob wells, for example, normally do not produce more than 0.5psig (pounds per square inch of gas) of pressure. An example of therelationship of well quality as a function produced pressure would readas follows: A high quality well can produce at 0.25 psig and then becomea low quality well when the pressure is pulled down to 0 psig. A wellmay produce 100 MCF/Day at 0 psig at 950 BTU (British Thermal Units).That same well under a vacuum and could produce at 150 MCF/Day at −1psig and 750 BTU. In many gas fields 750 BTU is not considered pipelinequality, thus it would better to produce the lower volume at the higherBTU value.

In a natural gas (CBM) gathering system here multiple producing wellsare feeding into a closed system, a change in flow rate/producingpressure at any single well location will have an effect on theproducing pressure/volume/BTU of the other wells on the closed system ifthe blower/compressor is not adjusted accordingly to compensate for thechange. This becomes a problem when the quality (BTU) of each given wellis a function of the producing pressure (whether vacuum or positive)applied to the wellhead, and when the closed gathering system has a lownumber of wells with lower volumes. It is very advantageous to keep aprecise and constant suction pressure on each well location in theclosed system regardless of how much volume is produced from each wellbore.

Assume a gathering system having ten wells with each well producing 100MCF/Day produces a total of 1,000 MCF/Day. If five of the wells producean energy content of 990 BTU and five of the wells produce 850 the totalquality of the system would be 920 BTU. If one of the ten wells' qualitydeteriorates to only produce 750 BTU, then this will lower the totalquality the system. In order to optimize the production of the system, awell tender may opt to cut this well off or close it from productioninto the gathering system. The compressor station will still run 1,000MCF/Day, and it will simply make up the volume from the remaining 9wells in the gathering system. The well tender would then have to runback to the compressor and reduce the throughput of the compressor tocompensate for the field change (slut-in of the one of the wells thatonly produce 750 BTU). If the well tender does not reduce the throughputof the compressor, the added volume that the 9 remaining wells producewill cause them to reduce their quality and could cause the entiresystem to drop below acceptable BTU levels.

Additionally, a well tender could go to a well after reducing throughputof the compressor, and find that the BTU is 990, and want to add morevolume from that particular well. He would first open the throttle valve(to further open the well into the pipeline), then run to the compressorstation and increase the throughput of the machine, so that the rest ofthe wells on the gathering systems producing pressure would remainunchanged.

The present invention prevents field operators from running in circlesadjusting wells then running to the feed station to adjust pressure.

This invention provides for a method for maintaining the quality of gasproduced from a group of wells by maintaining a constant pressure. Aplurality of wells are provided. Each well is capable of producing adifferent individual quality and individual volume of gas, both of whichcan change over time and because other factors such is atmosphericpressure changes (the weather/rise and fall of the atmosphericpressure). The individual wells are connected via a gathering line. Thegathering line has a total quality of gas and a total volume of gas thatresults from the sum of the individual qualities and the individualvolumes obtained from the plurality of wells. A compressor is connectedto the gathering line and pulls/collects a volume of gas through thegathering line. The gas pulled/collected through the gathering line isobtained from the plurality of wells connected to the pipeline. Acontrol device (such as a valve) controls the volume of gas flowingthrough the gathering line, and thereby controls the pressure of the gasin the gathering line as well as the pressure of gas exiting thecompressor. The control device is set at one level to maintain a desiredpressure. The individual volume of a particular well is adjusted tomaximize the quality of gas produced. The adjustment of the volumeproduced from the individual well adjusts the total volume of gasavailable to the gathering line. The control device automaticallyadjusts in response to the change in the total volume of gas availableto the gathering lines so that both the desired pressure and the desiredquality of gas are maintained. If the pressure was not maintained, therewould either be additional or less volume pulled from the remainingwells. When additional or less volume is pulled from the remainingwells, it can lower the quality of the individual wells.

The object of this invention is to maintain the quality of gas producedfrom a group of wells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a plurality of wells connected to a gatheringline, a compressor and a transmission line.

DETAILED DESCRIPTION Examples and Explanatory Definitions

Maintaining the quality of gas produced from a group of wells—Keepingthe total BTU of a group of gases above certain number, or keeping thetotal BTU of a group of gases within 200 BTUs of a desired number,preferably within 100 BTUs of a desired number, or most preferably,within 50 BTUs of a desired BTU.

Quality of gas—The energy content. For Example, British Thermal Units(“BTU”), is a unit for measuring heat quantity in the customary systemof English units of measurement, equal to the amount of heat required toraise the temperature of one pound of water at its maximum density[which occurs at a temperature of 39.1 degrees Fahrenheit (° F.)] by 1°F. The BTU may also be defined for the temperature difference between59° F. and 60° F. One BTU is approximately equivalent to the following:251.9 calories; 778.26 foot-pounds; 1055 joules; 107.5 kilogram-meters;0.0002928 kilowatt-hours. A pound (0.454 kilogram) of good coal whenburned should yield 14,000 to 15,000 Btu; a pound of gasoline or otherfuel oil, approximately 19,000 BTU.

Maintaining a constant pressure—keeping the pressure within a definedbandwidth.

A plurality of wells—More than 1 well.

Capable of producing a different individual quality and individualvolume of gas—Each well could produce a different quality of gas atdifferent flow rates.

Individual volume of gas—The volume of gas produced by a single well.

Individual quality of gas—The energy content value of the gas producedfrom a single well, for example, 950 BTU

A gathering line—A pipeline that individual wells are connected to. Thispipeline carries the total amount of gas produced bu the individualwells.

Connected to the plurality of wells—Attached via a pipeline to the wellsso that gas can flow from the wells.

Total quality—The energy content of the gas flowing through thegathering line.

A compressor—A device which changes the pressure of a volume of gas. Itcould be a blower. A machine used to supply air or other gases at anincreased pressure.

Examples of these devices include:

-   1. A Lamson Model 858 Blower;-   2. A Woodward Model 41G Blower; and-   3. A Allen Stewart SN#8315-7 Skid, Sullair Series PD10 Compressor.

A control device—A device which controls either the flow of gas througha pipeline, or the volume of gas into the pipeline. It could simply be avalve within the gathering line or within the recycle line of acompressor, variable frequency drive for the motor of the compressor, orloader valves for the compressor, or any other device or controller thatchanges the throughput of the machine. Examples of these devicesincluded:

-   1) Keystone—PIN: 2.0 920-723*K2HA2K2-BCDE-AYYYYY-YY April 2005    2″KEYSTONE FIGURE 920 TRIM 723, RESILIENT SEATED THIN DISC BUTTERFLY    VALVE. LUG STYLE CAST IRON BODY, 316 STAINLESS STEEL DISC & STEM,    BUNA SEAT MATERIAL. ASSEMBLED WITH A KEYSTONE EPI-6 ELECTRIC    ACTUATOR. 120 VAC POWER, WEATHERSPROOF ALUMINUM NEMIA 4/4X/7    HAZARDOUS AREA ENCLOSURE. OPEN & CLOSE TRAVEL LIMIT SWITCHES, 6 WATT    ANTI-CONDENSATION HEATER. SIDE MOUNTED MANUAL HANDWHEEL;-   2) PIN: 2.0 920-723*K2HH2K2-BCDE-CYYYYY-YY June 2005 2″KEYSTONE    FIGURE 920 TRIM 723, RESILIENT SEATED THIN DISC BUTTERFLY VALVE. LUG    STYLE CAST IRON BODY, 316 STAINLESS STEEL DISC & STEM, BUNA SEAT    MATERIAL. ASSEMBLED WITH A KEYSTONE EPI-6 ELECTRIC ACTUATOR. 24 VDC    POWER, WEATHERPROOF ALUMINUM NEMA 4/4X/7 HAZARDOUS AREA ENCLOSURE.    OPEN & CLOSE TRAVEL LIMIT SWITCHES, 5 WATT ANTI-CONDENSATION HEATER.    SIDE MOUNTED MANUAL HANDWHEEL; and    -   3) Valvcon—Model #VWXS300S2S24D (24 Volt DC Actuator 120 Volt AC        Models are used as well) Model #397-982 CI (ABZ Valve) May 2005.        *Note larger Keystone valves and actuators, along with        modulating actuators have been used with this system.

Setting the control device for a desired pressure—Inputting an optimumpressure value.

Desired pressure—A pressure when applied and remained constant to thegathering line, the quality and volume of gas at each individual well issaid to be at an optimum level.

Adjusting the individual volume of a particular well—Allowing eithermore or less gas produced from the well into the pipeline. An examplewould be to close the valve completely.

Maximize the total quality of the gas being produced—Trying to achievethe highest BTU value.

Automatically adjusting the control device—The control device changeswithout a human being taking any action.

Shut in—A state or period in which an oil or gas well has unusedcapacity, such as when the well is manually closed.

Adjusting the volume of gas automatically by an automated—The volume ofa particular gas changed without a human being taking any action. AnExample of such a device can be seen in application Ser. No. 11/264,477.

The total volume of gas going through the compressor—The volume of gastaken from the gathering line and compressed into the transmission line.

“Programmable logic controller”—A programmable logic controller, PLC, orprogrammable controller is a small computer used for automation ofreal-world processes, such as control of machinery on factory assemblylines. The PLC usually uses a microprocessor. The program is usuallycreated by a skilled technician at an industrial site, rather than aprofessional computer programmer. The program is stored inbattery-backed memory. The main difference from other computers is thespecial input/output arrangements. These connect the PLC to a process'sensors and actuators, PLCs read limit switches dual-level devices,temperature indicators and the positions of complex positioning systems.Some even use machine vision. On the actuator side, PLCs drive any kindof electric motor, pneumatic or hydraulic cylinders or diaphragms,magnetic relays or solenoids. The input/output arrangements may be builtinto a simple PLC, or the PLC may have external I/O modules attached toa proprietary computer network that plugs into the PLC. 1) Rel-tek PLCExpress (Gold) DX8884 Serial #1003 December 2004 2) Allen Bradley -Model # 1760-L12BWB February 2005 3) Allen Bradley - Model #1760-L18BWBEX June 2005 4) Telemecanique - Model # TWDMDA20DTK July 2005

Description

FIG. 1 shows a typical gathering system setup. Wells 2, 4, 6, 8, 10, 12,and 14 are connected to gathering line 16. Wells 2, 4, 6, 8, 10, 12, and14 are all capable of producing at different quality, volume, andpressure; therefore, each well has a individual volume and an individualquality. These individual volumes and qualities add up to give a totalvolume for the gathering line and an overall quality. Quality will nowbe referred to as BTU. A compressor 18 is connected to the gatheringline 16. The compressor pulls the gas from wells 2, 4, 6, 8, 10, 12 and14, and through the gathering line 16. Because of the closed system, thegas is under a pressure (or vacuum) as it travels through the gatheringline 16, into the compressor 18 and out the transmission line 20. Apressure transmitter 22 is connected to the gathering line 16. Pressuretransmitter 22 monitors the pressure in the gathering line 16.

The pressure transmitter 22 is connected to a programmable logiccontroller (“PLC”) 24. The PLC uses the information from the pressuretransmitter 22 to adjust control device 26. The control unit 26 canchange the throughput of the compressor to either increase pressure orto reduce pressure. This all happens automatically.

If a field person monitoring well 2 finds that it is only producing 650BTU, the field person may opt to shut in well 2. The control device 26,the PLC 24, and the pressure transmitter 22 work together toautomatically keep the pressure in gathering line 16 constant. Bykeeping the pressure constant the best BTU/flow) rate is maintained forthe group as their specific producing pressure will remain unchanged.Without the control device 26, the PLC 24 and the pressure transmitter22 the suction pressure in the gathering line would increase (in thecase of a vacuum state the vacuum would draw down deep, say −1 psig to−3 psig). The shortage of volume formerly produced from well 2 beingclosed, would be made up by Wells 4, 6, 8, 10, 12 and 14. The increasedsuction pressure and produced volume from wells 4, 6, 8, 10, 12, 14could cause the BTU values being produced from those wells to drop, thuscausing a drop in BTU value in the gathering line 16.

The above-described well system can be used in connection with the wellsystem described in application Ser. No. 11/264,477 the disclosure ofwhich is herein incorporated by reference. The shutting in of a singlewell in a gathering system having an automated system described inapplication Ser. No. 11/264,477, without the above described method,could cause the entire gather system to shut down.

Various changes could be made in the above construction and methodwithout departing from the scope of the invention as defined in theclaims below. It is intended that all matter contained in the abovedescription, as shown in the accompanying drawings, shall be interpretedas illustrative and not as a limitation.

1. A method for maintaining a quality of gas produced from a group ofwells by maintaining a constant gathering pressure comprising: a.providing a plurality of wells wherein each well is capable of producinga different individual quality and individual volume of gas; b.providing a gathering line that is connected to the plurality of wells,the gathering line has a total quality and a total volume of gas thatresults from the sum of the individual qualities and the individualvolumes of the wells; c. providing a compressor that is connected to thegathering line, the compressor gathers and compresses the total volumeof gas from the gathering line and introduces the compressed volume ofgas to the transmission line; d. providing a control device thatcontrols the total volume of gas going through the compressor therebycontrolling the pressure of the gas in the gathering line; e. settingthe control device for a desired pressure; f. adjusting the individualvolume of gas produced by a particular well in order to maximize thetotal quality of the gas being produced, thereby adjusting the volume ofgas available to the gathering line; and g. automatically adjusting thecontrol device in response to the change in the volume of gas availableto the gathering line so that the desired pressure is maintained andthereby maintaining the quality of gas in the gathering line.
 2. Themethod as recited in claim 1 wherein the particular well with the volumeadjusted is shut in.
 3. The method as recited in claim 1 wherein theadjusting the individual volume of a particular well is doneautomatically by air automated system.
 4. The method as recited in claim1 wherein automatically adjusting the control device is done by thecombination of a pressure transmitter and a programmable logiccontroller.